(i) Field of the Invention
The present invention relates to a method and apparatus for inhibiting scaling in an electrodeionization system or in a combined reverse osmosis/electrodeionization system for water treatment and, more particularly, for increasing tolerance to hardness in the feed water to an electrodeionization unit to inhibit precipitation of metal cations contained in the feed water and for increasing efficiency of the electrodeionization system.
(ii) Description of the Related Art
The purification of liquid has become of great interest in many industries. In particular, pure water is used for many industrial purposes such as, in processes for producing semiconductor chips, in power plants, in the petro chemical industry and for many other purposes.
Ion exchange resins, reverse osmosis filtration and electrodialysis techniques have been used to reduce the concentration of ions in a liquid.
Electrodeionization apparatus have recently been used with more frequency to reduce the concentration of ions in a liquid. The term "electrodeionization" generally refers to an apparatus and a process for purifying liquids which combine ion exchange resins, ion exchange membranes and electricity to purify the liquids. An electrodeionization module comprises alternating arrangements of cation permeable alternating compartments, there is provided ion exchange resin beads. Those compartments are known as diluting compartments. The compartments which generally do not contain ion exchange resin are known as the concentrating compartments. Ions migrate from the diluting compartments through ion exchange beads and ion permeable membranes into the concentrating compartments by the introduction of current. The liquid flowing through the concentrating compartments is discarded or partially recycled and the purified liquid flowing through the diluting compartments is recovered as demineralized liquid product.
Scaling of electrodeionization equipment is of particular concern as it reduces membrane efficiencies and fouls electrode surfaces. Scaling has been found to occur in localized regions of electrodeionization equipment, and particularly those where high pH is typically present. Such regions include those on the surface of the concentrate-chamber side of anion exchange membranes, due to the flux of hydroxyl ions resulting from the regenerative water splitting process in the diluting chambers. Localized regions of high pH are also typically present on the cathode surface due to the evolution of hydrogen gas and concomitant production of hydroxyl ion according to the cathodic electrode reaction: EQU 2e-+2H.sub.2 O=H.sub.2 (gas)+2OH--
These localized regions of high pH provide conditions under which scales harmful to the performance of the electrodeionization device can form. Generally, these scales form in the presence of polyvalent metal cations such as Ca.sup.2+, Mg.sup.2+, Sr.sup.2+, Ba.sup.2+, Fe.sup.3+, Al.sup.3+ and the like which can precipitate under local high pH conditions as hydroxides, sulphates, phosphates, oxides and carbonates, when carbonate, bicarbonate or carbon dioxide are present and as mixed oxides such as spinels, mixed carbonates and fluorides, when fluoride ions are present. Due to the low solubility products of these compounds, even trace amounts of these metal cations and counter anions in the concentrate streams will be sufficient to cause undesirable precipitation.
In reverse osmosis, as water passes through the membrane driven by the pressure difference between concentrate and permeate streams, the concentrate stream becomes progressively more concentrated and the solubility limit of salts of the dissolved ions can be exceeded, leading to precipitation of CaCO.sub.3 and other solids as scale. This mechanism of scale formation is fundamentally different from that in electrodeionization where the anion membranes surfaces (concentrate side) are actively maintained at a high pH due to the migration of hydroxyl ions from water splitting in the diluting chambers.
Antiscalants are used to prevent growth of such scales. These act by a number of mechanisms, including: a) inhibiting the nucleation of scale particles, usually with a sub-stoichiometric amount of antiscalant compared with scale forming ions; b) inhibiting the growth of scale particles, usually with a sub-stoichiometric amount of antiscalant compared with scale forming ions; c) solution complexing of the ions of the scale, thereby lowering the thermodynamic tendency (Gibbs energy change) for scale to form, and usually using a stoichiometric amount of antiscalant (chelating agent) compared with scale forming ions. Due to the relatively high concentration of ions in typical reverse osmosis feed water, effects a) and b) are usually effected by the introduction of antiscalant to the reverse osmosis feed. Lowering of the reverse osmosis feed pH is also commonly practised. The net effect is to permit operation of reverse osmosis systems at higher recovery than otherwise possible.
Although it is known to add antiscalants to reverse osmosis, it is not conventional practice to add antiscalants to electrodeionization cells for reducing scale formation. A paper entitled Studies on Polarity Reversal with Continuous Deionization by Yoram Oren et al. published in Desalination, 86 (1992) 155-172 by Elsevier Science Publishers B.V., Amsterdam, states that scale formation in continuous deionization, i.e., electrodeionization, can be minimized by reducing the concentration of calcium and magnesium (softening) or acidification which reduces pH in sensitive areas, or addition of antiscalants to form complexes with the calcium or magnesium ions or to delay precipitation. However, it is further stated that all such solutions add undesirable chemicals to the water and require equipment to introduce the chemicals.